The present invention relates to a flexible two-way telecommunication system, particularly, but not exclusively, to a cellular radio system such as a Universal Mobile Telephone System (UMTS).
A radio interface for UMTS is still at the conceptual stage but system designers are now beginning to specify a number of requirements, for example the ability to support a wide range of bit rates (8 kbps-2 Mbps) with high spectral efficiency are two of the most important. Other aspects which will have to be addressed are the flexible deployment of base-stations with the minimum of frequency planning, and the need to support real time services.
The radio interface should provide sufficient frequency diversity to mitigate the effects of multi-path propagation. Time diversity could be used but this is not very effective for stationary or slow moving terminals. Antenna diversity may also be employed, particularly at base stations.
Possible techniques include frequency hopping or broadband modulation. In the latter case a single broadband carrier would require the use of an equaliser at the receiver. Direct Sequence Spread Spectrum or Multi-Carrier Modulation could also be applied, requiring a Rake receiver or FFT demodulator respectively.
Norihiko Morinaga et al in an article entitled xe2x80x9cNew Concepts and Technologies for Achieving Highly Reliable and High-Capacity Multimedia Wireless Communications Systemsxe2x80x9d IEEE Communications Magazine, January 1997, pages 34 to 40, discuss the requirement for a highly reliable and a high capacity digital system for use as the third or fourth generation of mobile cellular telecommunications. In particular the merits of system design concepts and techniques for achieving highly reliable and high-capacity multimedia wireless communications using time division multiple access (TDMA) and code division multiple access (CDMA) systems are discussed. With respect to TDMA, this article proposes that a TDMAxe2x80x94based adaptive modulation with dynamic channel assignment is effective to achieve a high capacity microcellular system. The authors remark that broad bandwidth CDMA presents problems of severe interchip interference and of synchronisation at the receiver. As a consequence they examine slow frequency and OFDM (orthogonal frequency division multiplex) spread spectrum systems and remark that they will be a candidate for high rate transmissions in submillimeterxe2x80x94or millimeterxe2x80x94wave transmissions. Multicarrier CDMA systems are also considered but the conclusion was reached that single carrier CDMA systems were preferable. This article does not disclose how a flexible system using TDMA and CDMA techniques may be implemented.
An object of the present invention is to optimise the method of transmission of a signal to the conditions surrounding the receiving environment.
According to one aspect of the present invention there is provided a method of transmitting a signal between a primary station and a secondary station, each said station comprising transceiving means and control means, at least the control means of the primary station being capable of determining the quality of a signal received on an uplink, characterised by modulating a signal to be transmitted on a downlink and by selecting the characteristics of the transmission in response to information concerning the quality of the uplink and/or downlink.
According to another aspect of the present invention there is provided a two-way telecommunications system comprising a primary station and at least one secondary station comprising transceiving means, the primary station comprising transmitting means for transmitting signals on a downlink, receiving means for receiving signals transmitted by the transceiving means of the secondary station on an uplink and control means, said control means including means for determining the quality of a signal received by the receiving means from the uplink and means for modulating a signal to be transmitted on the downlink, said modulating means selecting the characteristics of the uplink and/or downlink transmission in response to information obtained concerning the quality of the uplink and/or downlink.
Other factors which may be taken into account when selecting the said characteristics include application requirements such as bit rate, bit error rate (BER) and end-to-end delay.